One of the criticisms I've seen leveled at Railgun S is that Mikoto passes up involving her friends. Evirus says:

For example, Mikoto spends most of the first cour of Railgun S sneaking around rather than enlisting the help of her roommate, the teleporter.

I can't say that Evirus is wrong because it's up to the show itself to convince its audience that events make sense and clearly the show hasn't sold Evirus on this. But what I can say is that the show did sell me on Mikoto's actions.

(There are about to be spoilers.)

First off, let's note that Mikoto does hardly any sneaking around in the first cour. She does about 90% of her work in a phone booth (and might have managed to do all of it if she wasn't impatient, but then if she had there wouldn't be much of a show) and much of the rest of it seems to have been less 'sneaking around' and more 'walking in casually'. In short, for almost all of the time she steamrollers everything in sight. The only time she actually needs any help happens when actual opponents appear (in the only actual fight) and this catches Mikoto completely by surprise when it happens.

(The show doesn't bother spending much time showing her steamroller stuff because it kind of lacks excitement.)

Beyond this, the show has sold me on a collection of reasons that Mikoto doesn't and can't involve her friends, especially Kuroko. In no particular order:

• Mikoto repeatedly talks about this being her fault and her problem to deal with. This is somewhat irrational but the show sells me on it being her heartfelt attitude, due in part to the horror of the situation.

• The situation is genuinely horrible (and it only gets worse as it goes along). Mikoto sees someone die in front of her and finds out that thousands of people have been systematically killed, with more happening every day. This is a huge trauma to dump on your middle school friends who are enjoying an innocent life in the sunshine. I can completely see not wanting to drag them into the nightmare.

• Kuroko is effectively a member of the police and Mikoto is busy doing all sorts of lawbreaking. It's at least uncool to ask your police buddy to help you commit a crime; you've put them in a really awkward spot no matter what they choose.

• Mikoto is not actually a member of Judgment and Kuroko has been shown as repeatedly trying to keep her from getting involved in Judgment operations (generally unsuccessfully, which results in Kuroko sighing a lot). It thus seems very likely that if Mikoto brought the whole problem to Kuroko officially she would immediately get sidelined (quite possibly very firmly, as Anti-Skill moved in). Mikoto is not exactly a sideline girl.

  (Of course, what we know in the first cour and what Mikoto finds out later is that going to the authorities wouldn't help anyways. Academic City is fundamentally corrupt and brutal.)

• If confronted by a choice between friendship (to Mikoto) and duty (to Judgment), Kuroko would probably choose duty. I say this because shortly after the end of the first cour (if I remember episode numbers right) the situation actually gets so bad that Mikoto probes Kuroko about this (phrased as a hypothetical) and Kuroko gives an unhesitating answer. Mikoto doesn't seem surprised.

(Mikoto actually seemed relieved, which felt right for me. Kuroko's answer meant that Mikoto didn't have to even consider dragging her friend into the darkness with her. Of course Kuroko might have made a different choice in a non-hypothetical situation.)